1. Technical Field
This invention relates generally to a method for using a variable valve actuation system to control the temperature and space velocity of exhaust gases delivered to an aftertreatment system in a diesel engine, and more particularly to a method for using a variable valve actuation system to optimize the conversion efficiency of exhaust gas aftertreatment devices over substantially all of a diesel engine operating range.
2. Background Art
Worldwide emissions regulations slated for introduction during the next five to ten years will require that diesel engines be equipped with some form of exhaust aftertreatment device, forcing the diesel industry into utilizing catalyst and emissions “trap” technologies not widely used heretofore for diesel engines. For optimum efficiency, these technologies, e.g., lean-NOx catalysts, lean NOx traps, and particulate traps, require operating temperatures outside of the temperature range available for a significant portion of a diesel engine operating range.
Several approaches have been proposed to control exhaust gas temperature in spark-ignition engines. For example, U.S. Pat. No. 6,029,626 granted Feb. 29, 2000 to Claus Bruestle for ULEV CONCEPT FOR HIGH-PERFORMANCE ENGINES proposes using variable valve timing to reduce unburned hydrocarbon emissions. Unburned hydrocarbon emissions are a pollutant commonly associated with spark ignited engines. U.S. Pat. No. 5,398,502 granted Mar. 21, 1995 to Kenzo Watanabe for a SYSTEM FOR CONTROLLING A VALVE MECHANISM FOR AN INTERNAL COMBUSTION ENGINE is also directed to exhaust gas temperature regulation in spark ignition engines. More specifically, U.S. Pat. No. 5,398,502 proposes the use of variable valve timing to control exhaust temperature as a strategy to increase exhaust gas temperature for fast catalytic converter lightoff during engine cold-starts. This patent further proposes exhaust gas temperature control when it is deemed that catalyst damage may occur due to over-heating.
Perhaps of most concern to the diesel engine industry are the proposed reductions in NOx emissions, because they are the most difficult to mitigate from the exhaust stream of fuel-lean combustion. Variations in engine airflow at constant fueling rates, typical in diesel engines, create a substantial range of exhaust gas temperatures and mass flow rates. One of the most promising technologies for NOx reduction in diesel engines is the “Lean NOx Trap” (LNT). However, lean NOx traps, like other catalytic-based aftertreatment devices, are most effective when the temperature and mass flow rate of exhaust gases passing through the aftertreatment device are maintained within limited ranges.
The present invention is directed to overcoming the problems associated with exhaust gas temperature and mass airflow control in diesel engines. It is desirable to provide a method for controlling the temperature and/or mass flow rate of exhaust gases over the entire functional range of a diesel engine to assure the optimal effectiveness of an aftertreatment device during both transient and steady state operation.